In a combine harvester-thresher the grain being harvested is threshed and then fed to shaking screens above grain and chaffer sieves. A winnowing blower directs a strong current of air horizontally across these two sieves and thereby separates the chaff from the heavier grain. In this manner virtually all of the grain is recovered from the crop and the stalks are left behind the combine in a neat windrow. Such a combine harvester-thresher is described on pages 432 ff. of THE WAY THINGS WORK (Simon & Schuster: 1967) and U.S. Pat. No. 3,566,880.
The winnowing blower of such a combine typically comprises an axially elongated housing having at one or both of its ends a suction fan and provided with a plenum passage extending the full axial length of the housing and opening radially therefrom. The fans draw air in through the inlets at the ends of the housing and create a high-pressure zone within the housing. This air is forced out of the plenum passage and over the sieves. Since the combine is a relatively wide machine, the chaffer and grain sieves are also relatively wide. Thus the current of air passed over them to winnow the grain must be of approximately the same strength across the full width of the machine. In the typical system the pressure near the ends of the blower, that is to the lateral side of the combine, is substantially less than at the center.
It has been suggested to even out this air stream by providing at the outlet end of the plenum passage a perforated plate having substantially larger holes toward the sides than at the center. Such an arrangement not only considerably reduces the efficiency of the blower, but is also subject to clogging. Thus it is necessary in such an arrangement to provide an oversized blower to compensate for the decreased efficiency and periodically to open the blower up and clean the distribution plate.
In another arrangement it has been sugggested to provide several distribution tubes within the housing of the blower. These tubes are formed basically as elbows having one end opening adjacent a respective fan and another end in the plenum at a position determined by the desired distribution. Such arrangements are relatively complicated and must be engineered with great precision in order to obtain the desired even effect.
It is also known to provide the fan completely externally of the blower housing and to use a complicated conduit arrangement with a manifold at the housing in order to insure even air distribution therein. Such an arrangement is relatively bulky and extremely complex, considerably raising te cost of the combine.
This air-distribution problem is worsened in the average combine where the amount of crop delivered to the sides of the shaking screen and, therefore, to the sides of the grain and chaffer sieves is greater than at the middle. On the contrary an average blower delivers a greater quantity of air to the middle than toward the end so that valuable grain is lost through ineffective winnowing or excessive amounts of chaff are present in the grain.